Xylem hydraulic impairment due to stronger drought stress accounts for the reduced crown vitality and growth performance of Mongolian pine trees in higher-density plantations

Abstract

Forest ecosystems worldwide are witnessing accelerated tree mortality caused by intensified drought under climate change. Tree density emerges as a pivotal factor affecting the stability of forests in water-limited areas; however, the physiological mechanisms by which stand density affects tree resilience to drought and forest stability remain elusive. Here, we conducted a comprehensive comparative analysis of Mongolian pine plantations with different densities (low, medium, and high) in a typical water-limited area of northern China. Employing a multi-faceted approach that integrates dendroclimatic analysis, individual tree-level physiological measurements, and forest canopy-level stress regime estimates based on drone-borne hyperspectral imagery, we sought to reveal the physiological mechanisms underlying the differences in growth vigor between Mongolian pine trees of different stand densities. We found that stand density had a strong influence on the performance of Mongolian pine plantations, with trees of higher-density stands exhibiting significantly lower radial growth rates and physiological traits reflecting stronger drought stress at both the individual and canopy levels. The dendroclimatic analysis underscored moisture as the primary environmental determinant for Mongolian pine growth in the study site. Comparisons between the selected relatively dry and wet years demonstrated that increased climate aridity exacerbated the density effect on hydraulic functioning. Across the stand densities and dry-wet climatic conditions, tree growth sensitivity to climatic variability linearly increased with leaf water status and xylem hydraulic efficiency but decreased with stem xylem embolism, portending that hydraulic impairment accounts for the diminished growth performance in denser stands. These findings highlight that stand density may modulate xylem hydraulic functionality by imposing on soil water availability and tree water status, thereby determining forest performance in water-limited areas. This research provides insights for sustainability-focused forestry management in drought-prone regions, advocating for tree density reduction that prioritizes xylem hydraulics to mitigate the negative impacts of drought on forest ecosystems.